Regression: broken nullability after update to v3.9.0

Empieza ejecutando la reproducción de C# proporcionada mediante SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0) y compara las salidas de v3.7.0 y v3.9.0. Rastrea la serialización de esquemas de OpenAPI 3.0 para JsonSchemaType.Null en additionalProperties y un esquema derivado de allOf; se considera terminado cuando nullable se conserva en ambos casos sin cambiar la salida mostrada para los demás campos.

Describe the bug
After updating from v3.7.0 to v3.9.0, the nullable property disappears when writing an OpenAPI 3.0 document.

Repro steps

using Microsoft.OpenApi;

var metaSchema = new OpenApiSchema
{
    Type = JsonSchemaType.Object,
    AdditionalProperties = new OpenApiSchema
    {
        Type = JsonSchemaType.Null
    }
};

var baseSchema = new OpenApiSchema
{
    Type = JsonSchemaType.Object,
    Id = "base",
    Properties = new Dictionary<string, IOpenApiSchema>
    {
        ["id"] = new OpenApiSchema
        {
            Type = JsonSchemaType.String
        }
    }
};

var derivedSchema = new OpenApiSchema
{
    Type = JsonSchemaType.Null,
    AllOf = new List<IOpenApiSchema>
    {
        new OpenApiSchemaReference(baseSchema.Id)
    },
    Properties = new Dictionary<string, IOpenApiSchema>
    {
        ["kind"] = new OpenApiSchema
        {
            Type = JsonSchemaType.String
        }
    }
};

var doc = new OpenApiDocument();
doc.Components ??= new OpenApiComponents();
doc.Components.Schemas ??= new Dictionary<string, IOpenApiSchema>();
doc.Components.Schemas.Add("meta", metaSchema);
doc.Components.Schemas.Add("base", baseSchema);
doc.Components.Schemas.Add("derived", derivedSchema);

var result = await doc.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0);
Console.WriteLine(result);

Using v3.7.0, this prints:

{
  "openapi": "3.0.4",
  "info": { },
  "paths": { },
  "components": {
    "schemas": {
      "meta": {
        "type": "object",
        "additionalProperties": {
          "nullable": true
        }
      },
      "base": {
        "type": "object",
        "properties": {
          "id": {
            "type": "string"
          }
        }
      },
      "derived": {
        "allOf": [
          {
            "$ref": "#/components/schemas/base"
          }
        ],
        "properties": {
          "kind": {
            "type": "string"
          }
        },
        "nullable": true
      }
    }
  }
}

Using v3.9.0, this prints:

{
  "openapi": "3.0.4",
  "info": { },
  "paths": { },
  "components": {
    "schemas": {
      "meta": {
        "type": "object",
        "additionalProperties": {
          "enum": [
            null
          ]
        }
      },
      "base": {
        "type": "object",
        "properties": {
          "id": {
            "type": "string"
          }
        }
      },
      "derived": {
        "allOf": [
          {
            "$ref": "#/components/schemas/base"
          }
        ],
        "properties": {
          "kind": {
            "type": "string"
          }
        }
      }
    }
  }
}

Screenshots/Code Snippets
View of the diff:

Originally reported at: https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/4065